Upon the completion of any improvement under sections 6117.01 to 6117.45, inclusive, of the Revised Code, the actual cost thereof shall be ascertained and to such actual cost shall be added an amount equal to the interest accrued and to accrue before the first installment of such assessment is collected upon certificates of indebtedness and upon bonds authorized by such sections or upon the contribution of the county to be paid from the assessment under an agreement pursuant to section 6121.13 of the Revised Code, between the county and the Ohio water development authority, for the construction of an improvement for which the county can levy assessments as provided in Chapter 6117 of the Revised Code, and the sum so arising, less the portion thereof to be paid by the county at large, shall be assessed against the lots and parcels of land within the assessment district found to be benefited by such improvement. For the purpose of paying the county sanitary engineer provided for in section 6117.01 of the Revised Code, and for paying his assistants and all of his other necessary expenses, and for the purpose of paying that part of the cost of the improvement to be paid by the county or of the interest to accrue thereon, the board of county commissioners may levy taxes in addition to all other taxes authorized by law. Such levy shall be subject to all the limitations provided by law upon the aggregate amount, rate, maximum rate, and combined maximum rate of taxation.